1. Field of the Invention
The present invention relates to an LCD, and more particularly to a fringe field switching mode LCD capable of realizing high transmittance.
2. Description of the Prior Art
A fringe field switching mode LCD has a counter electrode and a pixel electrode made of a transparent conductor and the spacing between them is smaller than that of upper and lower substrates to establish a fringe field above them.
FIG. 1 is a sectional view showing a fringe field switching mode LCD according to the prior art.
The fringe field switching mode LCD, as shown in FIG. 1, has lower and upper substrates 1 and 2 facing each other with a predetermined spacing between them and a liquid crystal (not shown) interposed between them.
The lower substrate 1 has counter electrodes 3 having a predetermined width and spaced a predetermined distance from each other, a gate insulation film 5 formed thereon, and a pixel electrode 7 formed on the gate insulation film 5. The counter electrodes 3 and the pixel electrode 7 are made of a transparent conductor, for example, ITO (indium tin oxide), and the spacing between them is smaller than the spacing between both substrates (i.e., cell gap).
The upper substrate 2 has a black matrix (not shown), a color matrix (not shown), and an overcoat film 4 formed thereon successively.
Orientation films (not shown) are formed on the uppermost part of the facing surface of the lower substrate 1 and on the surface of the facing surface of the upper substrate 2, respectively, to arrange the liquid crystal molecules in the liquid crystal layer in a batch mode before an electric field is established.
FIGS. 2 and 3 illustrate problems occurring in the prior art.
FIG. 2 is a graph showing V-T characteristics of a pixel electrode for each position in a fringe field switching mode LCD according to the prior art, wherein a refers to V-T curve corresponding to the average of transmittance values from the center to the edge of the pixel electrode, c refers to V-T curve at the center of the pixel electrode, e refers to V-T curve at the edge of the pixel electrode, and Vop in the abscissa refers to optimized driving voltage value. FIG. 3 is a graph showing Δnd-T characteristics of a pixel electrode for each position in a fringe field switching mode LCD according to the prior art, wherein a refers to Δnd-T curve corresponding to the average of transmittance values from the center to the edge of the pixel electrode, c refers to Δnd-T curve at the center of the pixel electrode, and e refers to Δnd-T curve at the edge of the pixel electrode.
The fringe field switching mode LCD according to the prior art exhibits different electro-optical characteristics between the center and edge of the pixel electrode 7. As shown in FIG. 2, particularly, the V-T curve (refer to c) at the center of the pixel electrode is similar to that of an in-plane switching (IPS) LCD and the V-T curve (refer to e) at the edge of the pixel electrode is similar to that of a low-twisted TN. As a result, the driving voltage at the center of the pixel electrode is larger than that at the edge of the pixel electrode. It is then impossible to obtain maximum transmittance in both positions of the center and edge of the pixel electrode. This decreases the transmittance.
It is known as a result of simulation that the optimized Δnd (phase delay value) at the center and edge of the pixel electrode is 0.36 and 0.440, respectively, as shown in FIG. 3. However, the Δnd in the fringe field switching mode LCD according to the prior art is 0.38, which is a mean value of both. As such, the conventional fringe field switching mode LCD is not optimized in terms of Δnd either.